The present invention generally relates to optical receivers and more particularly to an optical receiver having a photodetector circuit that employs an avalanche photodiode.
The commercial transceivers that are typically employed in networks generally incorporate a photodiode or photodetector as an optical receiver. Presently, the best-known PIN photodiode detectors have a responsivity on the order of 1 amp/watt or less. Often times, however, better reception sensitivity is desired to overcome issues that result from relatively low transmitter power and signal losses related to the use of connectors. One potential solution is to increase the power of the transmitter. As commercially available transmitters typically have a power level less than xe2x88x923 dBm (0.5 mW) at 850 nm, the eye-safe power limit at this wavelength, solutions that simply increase the power level of the transmitter are not practicable due to concerns for safety.
A relatively expensive option is to switch to a different wavelength, such as 1300 nm or 1500 nm, where the eye-safe limits increase to about +7 dBm and +17 dBm, respectively.
Substitution of an avalanche photodiode for the PIN photodiode is known to provide better receiver sensitivity due to the electron-multiplication gain of the avalanche photodiode. Avalanche photodiodes are similar to PIN photodiodes except that they have an additional high-field region that provides electron multiplier gain by accelerating photo-generated carriers to sufficient energy to create additional electron-hole pairs by impact ionization that in turn receive sufficient energy to create more electron-hole pairs. Increasing the bias potential increases the potential across the high-field region and increases the amount of gain. The use of an avalanche photodiode, however, presents some problems, including the need to adjust or control the bias potential to maintain both constant response (e.g., over a range of temperatures) and consistent response (i.e., device-to-device).
One popular approach to control the bias of an avalanche photodiode is to measure the temperature proximate the avalanche photodiode and use the temperature measurement to control the avalanche photodiode bias via open loop control. Another known approach is to use a second, unilluminated (i.e., dark) avalanche photodiode on the same monolithic chip operating in breakdown to provide a temperature tracking voltage reference and to bias the first avalanche photodiode at a fixed potential below the reference. Unfortunately, both methods are relatively complex to implement. A less complicated method is to simply fix the bias potential at a voltage that provides a usable responsivity over the full range of variation (e.g., temperature). This results, however, in an output having a magnitude that varies not only with the magnitude of the optical input but also with other variables, such as temperature and sample differences.
Accordingly, there remains a need in the art for an improved photodetector circuit that overcomes the aforementioned problems.
In one preferred form, the present invention provides a photodetector circuit for processing an optical input signal having an input signal magnitude. The photodetector circuit includes a power supply having an output terminal, a load device having an input terminal, and an avalanche photodiode that is coupled between the output terminal and the input terminal, the avalanche photodiode being configured to process the optical input signal. The power supply has a characteristic load line associated with the generation of a bias potential across the avalanche photodiode. The characteristic load line includes a substantially horizontal portion, which supplies power to the avalanche photodiode with a substantially constant current, and a substantially vertical portion, which supplies power to the avalanche photodiode with a substantially constant voltage. The power supply is configured to change from the first portion of the characteristic load line to the second portion of the characteristic load line when the bias potential across the avalanche photodiode that is generated by the substantially constant current is less than the magnitude of the substantially constant voltage.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.